Various systems are actually used to clean up gases or to remove dust from gases, fabrics for instance such as Nomex fabrics which may be used up to about 400.degree. F., Teflon fabric which may be used up to about 425.degree. F., glass fibre which may be used up to 500.degree. F. All these are expensive and do require special care. In order to prevent any damage by heat for filtering gases having higher temperatures, a cooling step is necessary. On the other hand, cooling removes all the possibilities of being able to use the heat as an energy source. These fabrics are not easy to operate and sometimes, serious problems of maintenance are encountered, particularly when hot particles in the gases to be filtered reach these filters.
Other filtration apparatus are known, such as electrofilters. These normally are resistant to temperature as high as 1,200.degree. F. However, their cost is prohibitive: Their maintenance and operation are in general reliable but electrical installations and sophisticated controls are required.
Another group of filtering systems involves washing. This implies cooling of gases and again inability to use hot gases as an energy source. Their maintenance and operating costs are in general high because of the problems associated with the treatment of the water used for washing.